1. Field of the Invention
The present invention relates to a motor controller that controls operation of a synchronous motor by determining an electrical angle in a sensor-free manner, as well as to a method of controlling a motor.
2. Description of the Related Art
In a synchronous motor that flows multi-phase alternating currents through windings and rotates a rotor by, it is required to regulate the multi-phase alternating currents made to flow through the windings according to the position of the rotor, that is, according to the electrical angle, in order to obtain a desired rotational torque. An electrical angle sensor, for example, utilizing a Hall element, has conventionally been used to determine the electrical angle of the rotor.
Some techniques have been proposed to determine the electrical angle in a sensor-free manner, in order to improve the reliability simultaneously with reduction of the cost. For example, JAPANESE PATENT LAID-OPEN GAZETTE No. 7-177788 discloses such a technique. When the motor rotates at a relatively low speed, the procedure of this proposed technique applies a specific voltage between the windings of the motor and monitors a behavior of electric currents made to flow in response to the applied voltage, so as to determine the electrical angle.
The following describes one concrete procedure of the electrical angle determination. In the case of a salient pole-type permanent magnet motor where the magnetic resistance in a magnetic circuit changes according to the angle of the rotor, a variation in electrical angle of the rotor varies the inductance of the windings and changes the behavior of the electric currents under the condition that a voltage is applied to the windings. By way of example, referring to the graph of FIG. 9, the greater inductance decreases the electric current at the time point when a preset time period has elapsed since the application of the voltage. There is a theoretical relationship between the inductance and the value of electric current. The apparatus for determining the electrical angle filed by the applicant of the present invention has been developed by taking into account this characteristic. In the proposed apparatus for determining the electrical angle, the relationship between the inductance and the electrical angle is stored in advance in the form of a table. The technique measures electric currents made to flow in response to a voltage applied to the windings, calculates an inductance from the behavior of the observed electric currents, and refers to the table to read the electrical angle corresponding to the calculated inductance. In order to determine the electrical angle with a high accuracy in this apparatus, it is required to precisely grasp the relationship between the applied voltage and the behavior of the electric current made to flow in response to the voltage.
When this prior art technique is applied to determine the electrical angle in a sensor-free manner, a voltage is applied for the determination of the electrical angle as shown in FIG. 9. The torque voltage is originally applied to the windings of the motor, in order to make a flow of the electric current relating to the output of the torque. The voltage for determination and the torque voltage are applied by separate control processes.
The graph of FIG. 20 shows the timings of the torque control and the electrical angle determination. B1, B2, B3 in FIG. 20 denote the execution timings of the torque control. In the actual state, a voltage is applied at a specific duty in the respective divisions B1, B2, B3 according to the voltage command value. In the example of FIG. 20, the torque command value takes a fixed value and the motor current has a constant effective value iT. S1 in FIG. 20 represents the execution timing of the electrical angle determination. Since the voltage for determination is applied over the substantially whole period of the electrical angle determination, the division S1 in FIG. 20 is substantially synonymous with the time period of application of the voltage for determination.
As shown in FIG. 20, the torque control is executed at narrower intervals than those of the electrical angle determination. The torque control is carried out even in the course of the determination of the electrical angle (see the processing at the timings B5 through B8 in FIG. 20). The control process in this period, however, does not newly read the torque command value but applies the voltage based on the torque command value previously input. While the voltage for determination is applied, the torque voltage for outputting the torque does not change, but the motor current has a fixed value iT. When the voltage for determination is superposed upon the torque voltage, the motor current varies as shown by a curve iS in FIG. 20. Since the value of the motor current is fixed during the application of the voltage for determination, the electrical angle can be determined by measuring the variation in electric current iS corresponding to the voltage for determination, which is superposed upon the torque voltage.
The prior art technique discussed above may, however, fail in accurately determining the electrical angle according to the execution timings of the torque voltage control and the electrical angle determination. Especially when the required torque changes immediately before the execution of the electrical angle determination, the prior art technique results in significantly lowering the accuracy of the determination of the electrical angle.
The lower accuracy of the determination of the electrical angle may interfere with the smooth operation of the motor; for example, it may cause a discontinuous variation in torque output from the synchronous motor. The non-smooth operation results in lowering the driving efficiency of the motor.